Grommet assemblies are often used to pass a wire bundle through an aperture in a panel. In the automotive industry, wire bundles must often be passed through the bulkhead panel of a vehicle as it is being wired. A grommet assembly is desirable to protect the bundle from the sharp edges of the aperture through the sheet metal of the bulkhead panel. A common known grommet is a simple flexible cylindrical rubber member designed to pop fit through an aperture with a wire or a small wire bundle passing therethrough. In modern vehicles, however, such simple grommets are often unsuitable, as it is necessary to protect and support much larger and heavier wire bundles. Consequently, the trend is toward heavier grommet assemblies, often known as hardshell grommet assemblies, that are larger and capable of handling and accurately locating the larger wire bundles. These hardshell grommet assemblies generally include a grommet body with a passage through which the wire bundle extends, and may also provide for injecting a sealant material into the passage around the wire bundle. Some means is necessary to attach the heavier grommet body to the bulkhead panel overlying the aperture, as a pop fit will generally be insufficiently strong. Various means for attaching a hardshell grommet assembly are disclosed in patents assigned to the assignee of the present invention. U.S. Pat. No. 4,289,924 to Pearce et al shows a hardshell grommet assembly 10 that is attached to a bulkhead panel 14 overlying an aperture 12 by bolts or other fasteners that pass through holes in the bulkhead panel 14. An improved means for attaching a grommet assembly that requires no fasteners passing through the bulkhead panel is disclosed in U.S. Pat. No. 4,407,042 to Schramme et al. There, a grommet assembly 10 has a shank 38 that is inserted through the bulkhead panel aperture. Then, a U-shaped retainer 22 is pushed in from the side into tapered slots 54 in the shank 38 to attach and retain the grommet assembly 10. This assembly works well where there is enough access space around the perimeter of the bulkhead panel aperture to in fact allow the retainer to be used. However, the available access space may be limited by the presence of obstructions or other structures already attached to the bulkhead panel, rendering such a retainer unusable.